Electromechanical vibrators



" SR 1 QR 3a234n782 Feb. 15, 1966 p GROOTENHUIS 3,234,782

ELECTROMEGHANICAL VIBRATORS Filed Jan. 23, 1962 INVENTOR. PETER GRooTENHms ATTORNEYS.

United States Patent 3,234,782 ELECTROMECHANICAL VIBRATORS Peter Grootenhuis, London, England, assignor of one-half to Derritron Limited, London, England, a company of Great Britain Filed Jan. 23, 1962, Ser. No. 168,201 Claims priority, application Great Britain, Jan. 26, 1961, 3,154/ 61 4 Claims. (Cl. 7371.6)

This invention relates to moving coil electromechanical vibrators including a pot magnet having an inner pole piece surrounded by an annular outer pole piece alfording between them an annular air gap to receive a moving coil carried by a tubular support connected to an output member supported so as to be capable of axial vibration with the coil.

According to one aspect of the present invention, the output member is connected to one end of the tubular support and also has a number of webs lying generally in planes containing the axis and connected to parts of the support spaced from the said end, whilst one of the pole pieces is formed with corresponding grooves to accommodate the said webs.

The webs may be situated inside the tubular support, the grooves being formed in the inner pole piece. In one arrangement the webs extend along substantially the whole length of the tubular support. The output member may have a central boss extending into a central re cess in the inner pole piece and the webs may extend from the boss to the tubular support.

According to another aspect of the invention the vibrator includes a diaphragm connecting the output member to a stationary part of the vibrator, for example the outer pole piece or a part carried by it, and enclosing an air-tight chamber containing the moving coil. Means may be provided for circulating a cooling fluid such as air in a closed circuit through the said chamber to cool the vibrator. Alternatively or in addition means may be provided for maintaining a superatmospheric pressure in the chamber to balance an axial force exerted on the output member by a load, for example by the weight of an object being tested.

The invention may be carried into practice in various ways but one specific embodiment will be described by way of example with reference to the accompanying drawings, in which:

FIGURE 1 is a central longitudinal section through an electromechanical vibrator embodying the invention, and

FIGURE 2 is a diagrammatic perspective view of the inner pole piece.

The vibrator comprises an energised pot magnet comprising a heavy circular base having a central upstanding boss 11 and an annular side wall 12 leaving between them an annular trough 13 in which is located an energising or field coil 14 to which a direct current is supplied to energise the magnet. Bolted to the top of the central boss by screws 15 is an inner pole piece which is surrounded by an annular outer pole piece 21 bolted to the side wall 12 by screws 22, the pole pieces leaving between them a narrow annular air gap 23. The pole faces may be copper plated.

Above the inner pole piece is an output member or load table 25 of generally circular form. The top of the table is fiat and may be provided with any suitable means such as threaded holes 26 for securing it to a load to be vibrated, whilst its underside has projecting from it a central boss 27, a peripheral skirt 28, an intermediate annular stiffening rib 29 and as described more fully below, six radial webs 30.

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The peripheral skirt 28 carries a tubular support 31 for a moving coil 32, which extends down into the air gap 23. The tubular support has at its upper edge an outwardly directed flange 33 which is bolted to the skirt 28 of the table (by screws not shown), and at its lower edge an inwardly directed flange 34 or reinforcing ring. The tubular support is of fibreglass with the moving coil, which consists of conductors either of circular or rectangular section wound coaxially, integrally embedded in its outer surface.

The six webs 30 on the underside of the table radiate generally in planes intersecting the axis from the central boss 27 to the tubular support 31, the lower edge of each web sloping down in a straight line from the bottom of the boss to the bottom of the support. Thus the cylindrical outer edge of each web is bonded to the coil support and in addition bolts 35 passing through the reinforcing ring 34 at the lower end of the coil support are threaded into holes in the lower edges of the webs. To accommodate the table the inner pole piece 20 is formed as shown in FIGURE 2 with a central recess 36 to accommodate the boss and a number of radiating grooves 37 to accommodate the webs. For example, in a vibrator having a coil 10 /2" in diameter and 4" long each web may have a width of /2" whilst the groove could have a width of /8". The depths of the annular rib and peripheral skirt are such as to terminate above the top of the inner pole piece.

The inner pole piece is formed with a spigot 38 located accurately in a shallow socket 39 in the top surface of the boss of the base and is secured by the screws 15 which pass through holes in the bottoms of the grooves 37 which accommodate the webs.

The table is supported by three diaphragms 40, 41 and 42 which enable it to move relatively freely in a direction parallel to the axis but locate it accurately to prevent transverse movement. The lowest diaphragm 42 has its inner periphery secured to the reinforcing ring 34 at the bottom of the tubular support, the middle diaphragm 41 has its inner periphery secured to the out-turned flange 33 at the upper end of the tubular support, whilst the upper diaphragm 40 has its inner periphery secured to the periphery of the table 25. The outer peripheries of the three diaphragms are secured to the underside of the outer pole piece, the upper side of the outer pole piece and the top of a ring 43 upstanding from and bolted to the top of the outer pole piece.

The uppermost diaphragm 40 is faced with a rubberised fabric and forms an airtight connection between the table and the outer pole piece so as to alford a closed air tight chamber containing the moving coil. The ring 43 carrying the upper diaphragm has in it one or more inlet ports 44 for air or other cooling fluid, whilst one or more outlet ports 45 are formed near the bottom of the side wall so that a cooling fluid can be circulated through the vibrator to pass down through the air gap and thence through the trough containing the energising coil to carry away heat. The inlet and outlet ports may communicate with ducts 46 and 47.

The middle and lower diaphragms have slots or holes in them to allow the passage of cooling air and if desired also to increase their flexibility.

A rubber butter 48 is secured on the bottom of the central recess in the inner pole piece to limit downward movement of the table and a number of rubber buffers (not shown) may be secured to the underside of the outer pole piece to be engaged by the lowermost diaphragm to limit upward movement. Alternatively or in addition an outwardly directed portion of the lower edge of the skirt of the table may be arranged to engage fixed buifers to limit upward movement.

This portion of the skirt of the table may also cooperate with a number of rubbing stops to prevent sideways movement due to imperfect application or use of the vibrator. Each such stop consists of a fixed pillar 51 in the form of a bolt screwed into the upper surface of the outer pole piece and surrounded by a hard rubber or plastic sleeve 52, having a low co-efiicient of dry friction, which can be engaged by a machined surface of the skirt of the table if the latter moves transversely. The upper edges of the pillars are conveniently connected together and to a further ring of pillars 53 by an annular platform 54 which serves to stiffen them and also carries a de-gaussing coil 55 to reduce and oppose any stray magnetic fields surrounding the table top. As a further safeguard against transverse movement the inner periphery of the bottom diaphragm is extended slightly inwards so as to be separated from the inner pole piece by a clearance less than that in the air gap.

The operation of the vibrator may be in accordance with known practice, but the construction described enables it to function satisfactorily at comparatively high frequencies due to the fact that the whole of the length of the moving coil is very rigidly secured to the table and the table itself is of very rigid construction. Thus the vibrator can be operated over a very wide frequency range without exciting the table into a resonant mode of vibration of its own. The lowest natural frequency of a table of given proportions depends upon the diameter, typical calculated values being as follows, taking into account the effect of the mass of the windings and coil former at the rim:

Diameter, inch: Frequency, c.p.s.

The lowest resonant frequency of a web is also high for the construction described, e.g. for a 10 inch diameter table it is calculated at 8,600 c.p.s.

It will also be appreciated that the vibrator is formed as a completely closed air-tight chamber with the exception of the inlet and outlet ports 44 and 45 for cooling air. Hence the cooling air may be circulated in a closed system at a pressure independent of the ambient pressure, so that the vibrator is suitable for operation within high altitude or climatic test chambers. In addition, the pressure within the chamber can be controlled so as to exert a predetermined upward force on the table so as to keep the table vibrating about a desired mean position irrespective of a steady load applied to it, for example due to the weight of an object being tested. Thus with the vibrator described it is possible to place upon the table heavy objects the weight of which could not be supported directly by the diaphragms.

What I claim as my invention and desire to secure by Letters Patent is:

1. A moving coil electromechanical vibrator comprising a pot magnet affording a single toroidal magnetic circuit and consisting of an inner pole piece and an annular outer pole piece projecting up coaxially from a base and forming an annular air gap between their upper ends, an exciting coil encircling the lower part of the inner pole piece substantially coaxially, the inner pole piece having a solid lower part and an upper part affording a plurality of grooves lying generally in planes containing the axis, a tubular support and a moving coil carried thereby and situated in the air gap, an output member comprising a table situated above the moving coil and directly connected to the upper end of the tubular coil support, and a plurality of webs lying generally in planes passing through the axis and each connected to the table and at least a part of the support adjacent the lower end of the coil with its lower edge sloping upwards and inwards from a point adjacent the lower end of the coil, the webs being received in the said grooves in the inner pole piece whereof the bottom of each groove slopes inwards and upwards close to the lower edge of the corresponding web, and means mounting the support and the output member so as to be capable of axial vibration.

Z. A moving coil electromechanical vibrator comprising a pot magnet affording a single toroidal magnetic circuit and consisting of an inner pole piece and an annular outer pole piece projecting up coaxially from a base and forming an annular air gap between their upper ends, an exciting coil encircling the lower part of the inner pole piece substantially coaxially, the inner pole piece having a solid lower part and an upper part atfording a central recess the bottom of which lies at a higher level than that of the air gap, and a plurality of grooves radiating from the said recess generally in planes containing the axis, a tubular support and a moving coil carried thereby and situated in the air gap, an output member comprising a table situated above the moving coil and directly connected to the upper end of the tubular coil support, a central boss projecting down from the table into the said central recess in the inner pole piece, and a plurality of webs radiating from the boss generally in planes passing through the axis and each connected to the boss, the table and at least a part of the support adjacent the lower end of the coil, with its lower edge sloping upwards and inwards from a point adjacent the lower end of the coil to the lower end of the boss, the webs being received in the said grooves in the inner pole piece whereof the bottom of each groove slopes upwards and inwards close to the lower edge of the corresponding web to the bottom of the recess, and means mounting the support and output member so as to be capable of axial vibration.

3. A moving coil electromechanical vibrator comprising a pot magnet affording a single toroidal magnetic circuit and consisting of an inner pole piece and an annular outer pole piece projecting up coaxially from a base and forming an annular air gap between their upper ends, an exciting coil encircling the lower part of the inner pole piece substantially coaxially, the inner pole piece having a solid lower part and an upper part affording a central downwardly tapering recess the bottom of which lies at a higher level than that of the air gap, and a plurality of grooves radiating from the said recess generally in planes containing the axis, a tubular support and a moving coil carried thereby and situated in the air gap, an output member comprising a table situated above the moving coil and directly connected to the upper end of the tubular coil support, a central downwardly tapering boss projecting down from the table into the said central recess in the inner pole piece, and a plurality of webs radiating from the boss generally in planes passing through the axis and each connected to the boss, the table and at least part of the support adjacent the lower end of the coil, with its lower edge sloping upwards and inwards from a point adjacent the lower end of the coil to the lower end of the boss, the Webs being received in the said grooves in the inner pole piece whereof the bottom of each groove slopes upwards and inwards close to the lower edge of the corresponding web to the bottom of the recess and means mounting the support and output member so as to be capable of axial vibration.

4. A moving coil electromechanical vibrator comprising a pot magnet affording a single toroidal magnetic circuit and consisting of an inner pole piece and an annular outer pole piece projecting up coaxially from a base and forming an annular air gap between their upper ends, an exciting coil encircling the lower part of the inner pole piece substantially coaxially, the inner pole piece having a solid lower part and an upper part affording a central downwardly tapering recess the bottom of which lies at a higher level than that of the air gap, and a plurality of grooves radiating from the said recess, generally in planes containing the axis, a tubular support and a moving coil carried thereby and situated in the air gap, an output memher comprising a table situated above the moving coil, and having an integral peripheral skirt projecting down from it and directly connected to the upper end of the tubular coil support, a central downwardly tapering boss projecting down from the table into the said central recess in the inner pole piece, and a plurality of webs radiating from the boss generally in planes passing through the axis, each of the said webs being integral with the table, the boss and the skirt and connected to at least a part of the support adjacent the lower end of the coil, with its lower edge sloping upwards and inwards from a point adjacent the lower end of the coil to the lower end of the boss, the webs being received in the said grooves in the inner pole piece, whereof the bottom of each groove slopes upwards and inwards close to the lower edge of the corresponding Web to the bottom of the recess, and means connected to opposite ends of the tubular support mounting the support and output member so as to be capable of axial vibration.

References Cited by the Examiner UNITED STATES PATENTS FOREIGN PATENTS 5/ 1958 Great Britain. 5/ 195 8 Great Britain. 12/ 1961 Great Britain. 12/ 1961 Great Britain.

15 RICHARD C. QUIESSER, Primary Examiner.

JOHN P. BEAUCHAMP, Examiner.

J. I. GILL, Assistant Examiner. 

1. A MOVING COIL ELECTROMECHANICAL VIBRATOR COMPRISING A POT MAGNET AFFORDING A SINGLE TOROIDAL MAGNETIC CIRCUIT AND CONSISTING OF AN INNER POLE PIECE AND AN ANNULAR OUTER POLE PIECE PROJECTING UP COAXIALLY FROM A BASE AND FORMING AN ANNULAR AIR GAP BETWEEN THEIR UPPER ENDS, AN EXCITING COIL ENCIRCLING THE LOWER PART OF THE INNER POLE PIECE SUBSTANTIALLY COAXIALLY, THE INNER POLE PIECE HAVING A SOLID LOWER PART AND AN UPPER PART AFFORDING A PLURALITY OF GROOVES LYING GENERALLY IN PLANES CONTAINING THE AXIS A TUBULAR SUPPORT AND A MOVING COIL CARRIED THEREBY AND SITUATED IN THE AIR GAP, AN OUTPUT MEMBER COMPRISED A TABLE SITUATED ABOVE THE MOVING COIL AND DIRECTLY CONNECTED TO THE UPPER END OF THE TUBULAR COIL SUPPORT, AND A PLURALITY OF WEBS LYING GENERALLY IN PLANES PASSING THROUGH THE AXIS AND EACH CONNECTED TO THE TABLE AND AT LEAST A PART OF THE SUPPORT ADJACENT THE LOWER END OF THE COIL WITH ITS LOWER EDGE SLOPING UPWARDS AND INWARDS FROM A POINT ADJACENT THE LOWER END OF THE COIL, THE WEBS BEING RECEIVED IN THE SAID GROOVES IN THE INNER POLE PIECE WHEREOF THE BOTTOM OF EACH GROOVE SLOPES INWARDS AND UPWARDS CLOSE TO THE LOWER EDGE OF THE CORRESPONDING WEB, AND MEANS MOUNTING THE SUPPORT AND THE OUTPUT MEMBER SO AS TO BE CAPABLE OF AXIAL VIBRATION. 